In a conference talk on “Direct optical vorticity probing”, 14th European Turbulence Conference, Sep. 1-4, 2013, Lyon, France, the inventors disclosed micro spheres of 70 μm diameter on average as particles for seeding a fluid flow to trace the motion of the fluid flow in two translational and three rotational directions. The micro spheres are illuminated by a collimated laser. The reflections from the mirrors within the micro spheres are registered by two cameras. The first camera tracks the translational motions of the mirrors. The second camera arranged in the focal plane of an objective registers the directions of the reflections from the mirrors independent from their positions. Thus, the mirror normal directions can be obtained from positions of light spots on the second camera. If the second camera is calibrated, a one-to-one mapping between pixel positions and mirror orientations can be obtained. The calibration is done using an array of mirrors whose orientations are known. When more than three successive normal directions of one mirror are known, a rotation rate of the mirror may be deduced.
The position of a small object in a field of view of an objective can be determined from the position of a light spot on a camera arranged in an image plane onto which the object is imaged by the objective. With only one objective and one camera, the position of the object can only be determined in the two spatial dimensions orthogonal to the optical axis of the objective. A further camera arranged in a further image plane onto which the object is imaged by a further objective whose optical axis points to the field of view in another direction than the optical axis of the first objective, i.e. a stereographic camera arrangement, allows for determining the position of the object in all three spatial dimensions.
It is known to provide an objective having a large aperture by means of a concave mirror arranged close to the field of view of the objective.
There still is a need of a method of and an apparatus for optically determining positions and orientations of mirrors in which the orientations of a plurality of mirrors in a field of view of an objective determined by means of a camera arranged in the focal plane of the objective can be assigned to the positions of these mirrors determined with another camera arranged in an image plane onto which the objective images the field of view, so that a plurality of images taken with both cameras, for example, allows for spatially resolved determining the vorticity of a fluid flow within or through the field of view seeded with the mirrors or deformations of a surface to which the mirrors are attached, or inside a transparent body in which the mirrors are embedded.